1. Field of the Invention
This invention relates to electrical devices.
2. Introduction to the Invention
Ser. No. 08/121,717 and its published International counterpart WO 94/01876 disclose circuit protection devices which comprise first and second laminar electrodes; a laminar PTC resistive element sandwiched between the electrodes; a third laminar conductive member which is secured to the same face of the PTC element as the second electrode but is separated therefrom; and a cross-conductor which passes through an aperture in the PTC element and connects the third conductive member and the first electrode. This permits connection to both electrodes from the same side of the device, so that the device can be connected flat on a printed circuit board, with the first electrode on top, without any need for leads. The resistive element preferably comprises a laminar element composed of a PTC conductive polymer. Preferably the device comprises an additional conductive member and an additional cross-conductor, so that the device is symmetrical and can be placed either way up on a circuit board. Ser. No. 08/242,916 and its published International counterpart WO 95/31816 describe improved devices of the kind described in Ser. No. 08/121,717 which include insulating members which prevent solder bridges between the conductive member and the adjacent electrode. Ser. No. 08/257,586 and its published International counterpart WO 95/34084 describe an improved method of making such devices. The entire disclosure of each of those U.S. applications and International Publications is incorporated herein by reference for all purposes.
There is a demand for circuit protection devices which occupy a very small area on a circuit board and which have a lower resistance than can be conveniently produced by the known methods. We have discovered, in accordance with the present invention, that two or more of the symmetrical devices described in Ser. No. 08/121,717 International Publication No. and WO 94/01876 can be easily and economically connected together to make a composite circuit protection device which is easy to install and which has lower resistance per unit area than the individual devices.
The composite protection devices of the invention comprise
(A) a laminar insulating member;
(B) a first laminar circuit protection device; and
(C) a second laminar circuit protection device;
each of the first and second laminar circuit protection devices comprising
(1) a first laminar electrode;
(2) a second laminar electrode;
(3) a laminar PTC resistive element which (i) exhibits PTC behavior, (ii) has a first face to which the first electrode is secured and an opposite second face to which the second electrode is secured, and (iii) defines first and second apertures which run between the first and second faces;
(4) a third laminar conductive member which (i) is secured to the second face of the PTC resistive element in the area of the first aperture, and (ii) is spaced apart from the second electrode;
(5) a fourth laminar conductive member which (i) is secured to the first face of the PTC resistive element in the area of the second aperture, and (ii) is spaced apart from the first electrode;
(6) a first transverse conductive member which
(a) lies within the first aperture defined by the PTC resistive element,
(b) runs between the first and second faces of the PTC element,
(c) is secured to the PTC element, and
(d) is physically and electrically connected to the first laminar electrode and to the third laminar conductive member, but is not connected to the second laminar electrode; and
(7) a second transverse conductive member which
(a) lies within the second aperture defined by the PTC resistive element,
(b) runs between the first and second faces of the PTC element,
(c) is secured to the PTC element, and
(d) is physically and electrically connected to the second laminar electrode and to the fourth laminar conductive member, but is not connected to the first laminar electrode;
the first and second laminar devices being physically secured together in a stacked configuration, with the laminar insulating member between them; and the devices being connected together electrically by interfacial electrical connections between adjacent electrodes and laminar conductive members so that when an electrical power supply is connected to (i) one of the electrodes and (ii) the third or fourth laminar member on the same face of the PTC resistive element as the electrode (i), the first and second laminar circuit protection devices are connected electrically in parallel.
In addition to the advantages set out above we have found that the power dissipation of such a composite device is not substantially different from the power dissipation of one of the devices alone. As a result, the composite device has a lower resistance for a given hold current (xe2x80x9chold currentxe2x80x9d is the largest current which can be passed through a device without causing it to trip). Furthermore, by appropriate sorting of the individual devices before they are assembled into composite devices, variations within a batch of composite devices can be reduced.